A fixed type constant velocity universal joint, which has a structure for transmitting a torque by torque transmitting balls arranged in wedge-like ball tracks and for retaining and guiding the torque transmitting balls by a cage guided along a spherical inner surface of an outer joint member and a spherical outer surface of an inner joint member, is also referred to as “Rzeppa joint” (sometimes referred to as “Birfield joint” or “ball-fixed joint”). The constant velocity universal joint is widely used as a connecting joint for a drive shaft and a propeller shaft of an automobile, specifically, as a joint arranged on a fixed side (wheel side) of a drive shaft of an automobile.
In the Rzeppa joint, the wedge-like ball tracks are formed while offsetting guide grooves of the outer joint member and guide grooves of the inner joint member from the joint center. For use which requires a large operating angle, such as for use in a drive shaft joint or the like (a large operating angle equal to or larger than a maximum operating angle 45° is necessary for use in a drive shaft), normally, the guide grooves of the outer joint member are offset from the joint center to a joint opening side along a joint central axis line by a predetermined distance, and the guide grooves of the inner joint member are offset from the joint center to a joint innermost side along the joint central axis line by a predetermined distance. In this case, the joint central axis line represents a straight line including an axial line of the outer joint member and an axial line of the inner joint member in a state in which the operating angle of the joint is 0°, a joint central plane represents a plane including a center of each of the torque transmitting balls and orthogonal to the joint central axis line, and a joint center represents an intersection between the joint central plane and the joint central axis line.
As the above-mentioned Rzeppa joint, one having a structure of being provided with six torque transmitting balls has been used as a technical standard for years and supported by many users in aspects of performance, reliability, and the like. In this context, one of the inventors of the present invention has developed and already proposed an eight-ball Rzeppa joint which is made highly efficient and fundamentally lightweight and compact while securing strength, load capacity, and durability equal to or higher than those of the six-ball Rzeppa joint as the technical standard (for example, JP 09-317783A).
Further, JP 04-228925A and JP 2002-541395A disclose that, in the six-ball Rzeppa joint, the center of each of the guide grooves is offset to the position spaced apart from the joint central axis line to the opposite side in the radial direction with respect to each of the guide grooves.
That is, in JP 04-228925A, the first guide grooves of the external joint member are formed of the opening side first guide grooves about joint centers a as centers and the inner side first guide grooves about points b as centers, which are offset from the joint centers a to the opposite side in the radial direction. The second guide grooves of the interior joint member are formed of the inner side second grooves about points c as centers, which are offset from the joint centers a to the inner side along a joint central axis line L1, and the opening side second guide grooves about points d as centers, which are offset from the points c to the still opposite side in the radial direction. With this structure, the groove depth of each of the inner first guide grooves of the external joint member is increased, and the thickness of the interior joint member is increased at parts corresponding to the opening side second guide grooves. Therefore, when the joint forms a large operating angle, the problem that the balls climb onto the inner first guide grooves of the external joint member so as to chip the edge portions of the grooves is eliminated, and the problem that the interior joint member is damaged owing to load applied from the balls is eliminated.
In JP 2002-541395A, a center B of each of the guide grooves of the bell (outer joint member) and a center C of each of the guide grooves of the nut (inner joint member) are offset to the positions separated from a diametrical plane (joint central plane) P to both the sides in the axial direction by a distance d1 and separated from a joint central axis line XY to the opposite side in the radial direction by a distance d2. With this structure, in a state in which the joint forms a maximum operating angle and the balls are extremely close to the inlet edge portion of each of the guide grooves of the bell, contact force between the balls and the guide grooves is reduced, whereby damages to the inlet edge portion of each of the guide grooves are prevented.